1. Field of the Invention
The present invention relates to an electronic device for providign protection from excessive current, and more particularly, to an electronic device serving as a support for an electronic element while also providing interrupt protection against excessive current flow.
2. Description of the Related Art
As one example of a conventional electronic device of this type, a thermistor device 1 which is used in a non-contact type starter for a compressor of a refrigerator is shown in FIG. 7.
Referring to FIG. 7, the thermistor device 1 comprises a disc-shaped thermistor 5 having a positive temperature coefficient (referred to as a PTC thermistor hereinafter), on both main surfaces of which, electrode films 4 are formed. In an outer packaging case 2 of a synthetic resin of the thermistor device 1, the PTC thermistor 5 is elastically supported between a pair of spring members 3, a respective one of which is in contact with each of the electrode films 4 of the thermistor 5, wherein each spring member 3 is formed by punching an elastic metal plate.
Referring to FIG. 8, each of the spring members 3 comprises a pair of V-shaped plate springs 6, wherein the center portions of the respective plate springs 6 are connected through a bridge portion 7 and both ends of the respective plate springs 6 are connected through respective bridge portions 8. Each spring member 3 is fixed on a tab-shaped connection terminal 9 so that the bridge portion 7 thereof is connected thereon by the spot welding method. A pair of connection terminals 9 are fixed on respective inner side surfaces 11 of the outer packaging case 2 which oppose each other. Both the electrode films 4 of the PTC thermistor 5 are pressed by both ends of both respecti,ve plate springs 6 of a pair of spring members 3, whereby the PTC thermistor 5 is elastically supported by the pair of spring members 3.
In the conventional thermistor device 1, if the PTC thermistor 5 starts a thermal runaway caused by an application of an abnormally high voltage or at the end of the life thereof, the current flowing through the PTC thermistor 5 increases steeply. Even in this case, since the connection terminal 9 and the spring member 3 have large current capacities, respectively, the electricity is supplied to the PTC thermistor 5 until the PTC thermistor 5 has been completely destroyed. In this case, the temperature of the thermistor device 1 increases up to about 1000.degree. C., and in the worst case, the thermistor device 1 may cath fire.
In order to solve the above-mentioned problems of the conventional thermistor device 1, for example, as shown in FIGS. 7 and 8, there has been proposed a thermistor device 1' having a spring member 3a which further comprises a plate portion 14 to be fixed on the connection terminal 9 by the spot welding method and a connection portion 15 for connecting the bridge portion 7 with the plate portion 14, in addition to a pair of plate springs 8 and the bridge portions 7 and 8 (See the Japanese utility model laid open publication (JP-U) No. 59-123301/1984), wherein the connection portion 15 has a width smaller than that of the plate portion 14.
In, the thermistor device 1', when the current flowing through the PTC thermistor 5 becomes larger than a predetermined threshold current after a thermal runaway is caused in the PTC thermistor 5, the connection portion 15 having the small width is fused (i.e., melted and/or cut). Therefore, the temperature of the PTC thermistor 5 can be prevented from becoming higher than a predetermined abnormal threshold temperature thereof.
However, if the connection portion 15 of the conventional thermistor device 1' shown in FIG. 9 has too small a width or too small a thickness, the connection portion 15 may be easily broken or be easily cut when an impact is applied thereto upon connecting the thermistor 1' with another device or a terminal of a printed circuit board. Therefore, it is necessary for the connection portion 15 to have a width larger than a predetermined necessary width and a thickness larger than a predetermined necessary thickness. Accordingly, it is necessary for the spring member 3a to have a relatively large current capacity. In this case, if a thermal runaway is caused in the PTC thermistor 5, it takes a long time to fuse the connection portion 15, resulting in the problem that the outer packaging case 2, etc., may be thermall damaged or destroyed.